The present invention relates to a device for adjusting electron beams in a cathode-ray tube and particularly the convergence and colour purity of a colour-picture tube comprising an electron gun with electrodes which is housed in the tube neck, which extends in the z-direction. The gain produces at least two electron beams lying in an x-z plane. The adjusting device includes a magnetic ring which is attached to the electron gun and is magnetizable from outside and whose two long sides and two short sides enclose an oblong area which is normal to the electron-beam plane and whose long axis (x-axis) lies in the electron-beam plane, while its short axis (y-axis) is perpendicular thereto.
A colour-picture tube with a device of this kind was shown at the "Electronica 1980" exhibition. The device itself is described in "Funkschau" 26/1980, page 57, in an article on that exhibition. The magnetic ring is made of an iron-cobalt-vanadium alloy and consists of two parts placed end to end leaving air gaps. It is fastened in the focusing electrode of the electron gun right in front. To adjust colour purity and static convergence, it is magnetized from outside, i.e., through the tube neck.
Each part of the magnetic ring has an inwardly directed curvature at the center of its long side. The purpose of such a curvature is described in German patent application No. P 30 03 197. It is to move all three electron beams horizontally, i.e., to the left or right, by equal values during colour-purity adjustment. To achieve this purpose, it was found to be advantageous if the distance between the long sides of the ring decreases from the outer portions toward the center. It may decrease uniformly from the edge toward the center, or an inward curvature toward the central electron beam may be provided.
With a device of the above kind it is possible to adjust electron beams in cathode-ray tubes and particularly in colour-picture tubes to achieve colour purity and static convergence. It is known, however, that magnetic fields not only deflect an electron beam but also influence the shape of the beam, i.e., the distribution of the electrons moving therein. DE-AS No. 11 33 838, for example, proposes to use magnetic bars in an electron-beam tube for beam shaping.